This invention relates to an electric element and to a toaster or other cooking apparatus (which term includes apparatus for reheating already cooked food) incorporating a plurality of such elements.
Conventionally an element for an upright toaster comprises a former of an electrically non conductive material and an electrically conductive tape wound continuously round the former with each run thereof extending between grooves in the opposite side edges of the board or each half of the board. The conductive tape is exposed except that one or more narrow strips of non-electrically conductive material are frequently located outside the tape running from top to bottom of the former, and spaced from the side edges, to limit the amount of outward bowing of the tape and to prevent the tape flying outwards if it breaks.
A toaster comprises a plurality of such elements which could be connected in parallel or in series. Because the conductive tape is exposed and can be easily touched by anything inserted into the toaster slots it is a requirement that the toaster must be double pole switched. This increases the cost of the toaster particularly if any electronics are incorporated in the toaster. It is also a requirement for upright toasters that an array of guard bars to prevent toast contacting the elements and which define the toaster slot are spaced at least 4 mm from the tape (since this is likely to bow outwardly when hot).
Conveyor toasters or other cooking apparatus in which elements are located horizontally conventionally also have live conducting tape or strip exposed, and suffer from uneven heating across the width of the apparatus. It has been proposed to use mica or micanite sheets as protective covers for the conductive tape. For example UK-A-2324694 shows a mica sheet placed over a heating element and retained by bent over metal tabs. UK-A286807 has insulated sheets on opposite sides of a heating element, kept in place by paper fasteners at the four corners. U.S. Pat. No. 4,745,855 has micanite covers over the heating element held in place by tabs along the top and bottom edges. These methods of attaching a cover are unsatisfactory. One object of the invention is to provide improved construction of such a heating element.
In one aspect, the present invention provides an electric heating element in which electrically conductive tape is wound on a non-electrically conductive former and the tape is completely covered by a cover or covers of electrically non-conductive but heat conductive material secured to the former by fastening means, characterised in that the or each cover is formed from two or more pieces, the cover pieces are secured to the former and each other by intermediate fastening means extending through the cover or covers at at least two spaced positions intermediate the sides and not contacting the tape, and by fastening means extending at spaced positions along two opposite edges of the former.
The fastening means are preferably rivets extending at spaced intervals down the vertical centre line and at spaced intervals down the side edges. Alternatively the fastening means may be staples, similarly arranged Using such elements in a toaster or other cooking apparatus is safer because the electrically conductive tape cannot be touched so as to make electrical contact by a knife or finger or other object inserted into the toaster or the like. This allows the apparatus to be single pole switched. In addition the number of guard bars in an upright toaster can be reduced because the toast in the slot cannot touch the conductive tape. In addition the arrangement increases the life of the element considerably because most failures are caused by bits of bread, toast, currants or other ingredients falling off and touching the tape. This causes a burn-out which may be explosive. Also instruments inserted can no longer break the tape. In addition because the tape cannot bow outwardly the space between the element and the guard bars in an upright toaster can be reduced.
Preferably the cover material is either mica or mica paper (a combination of mica and silicon resin), particularly that known as Micanite, either Moscovite or Phologopite, depending on the temperature requirements. We have found that with covers made of this material in a toasting apparatus, the toasting performance is improved for a particular heat input and is more even over the area of the toast. The former may be made of mica or Micanite so that the heat generated in the tape is absorbed by the former and cover material and then radiated evenly towards the toast.
The fastening means, such as eyelets, reduce bowing of the cover away from the former. The eyelets may hold in place washers of electrically insulating but heat conductive material to increase the area over which the covers are held to the former.
In one preferred form, where the former is made from a single piece of material, the tape starts adjacent one end of the former, is wound spirally towards the other end using alternate grooves, passes through a slot in the other end of the former to change direction, is wound spirally towards said one end using the remaining grooves and ends adjacent said one end of the former.
The former may be made in two parts, each with grooved upright edges with the tape wound first round one part, then the other part and the parts then secured together with abutting edges before connection of the covers. Each part may it self be preformed from one or more pieces connected along horizontal overlapping edges.
In a preferred form for an upright toaster, the former has a dimension in the range 130 to 150xc3x97160 to 180 mm (preferably 140xc3x97171 mm) and a thickness in the range 0.3 to 0.7 mm. Preferably each cover has a thickness in the range 0.1 to 0.3 mm.
Preferably the former and covers have convex rounded or sloped top edge corners corresponding to the shape of many loaves of bread, and to the end profile of the toaster. This allows the height of the former in the central portion to be greater.
The invention extends to an electric toaster or other apparatus including a plurality of such elements and which is preferably single pole switched. Preferably the elements are connected in parallel to an electrical supply. With such an arrangement, even if one element fails the remaining elements will continue to function. In addition each element can have a greater (or lesser) length of tape to produce the required watt density and thus produce more heat, giving faster toasting or heating.
For toasting the tape should be selected to produce a watt density equal to or less than 3 watts per cm2 or within the operating band of the tape. With conventional 80/20 nickel chrome alloy tape, and with parallel connection of the elements to a 230 volt supply, the tape for each element can have a length in the range 5 to 10 meters, preferably 7 to 9 meters, in order to achieve this.
For a conveyor toaster or other food heater where elements are located horizontally on either side of a continuous conveyor, the elements again provide safer and more even heating, particularly if the runs of tape at the centre of the element are more widely spaced than those adjacent the sides of the conveyor. The elements may also be used in other cooking apparatus such as grills or a pizza oven.